What is the X in X-mas?
A low-level interpreted language used in bootstrapping the BCPL compiler. The INTCODE machine has six control registers and eight functions. OCODE was used as the intermediate language.
["INTCODE - An Interpretive Machine Code for BCPL", M. Richards, Computer Lab, U Cambridge 1972].
["BCPL - The Language and its Compiler", Martin Richards & Colin Whitby-Stevens, Cambridge U Press 1979].